1. Field of the Invention
This invention is related to methods and systems for data recovery from damaged optical media, including enhancing signal recovery from optical media subjected to handling and non-archival environments.
2. Description of the Related Art
Optical data storage systems use light to write and read information. A schematic of typical read/write system is shown in FIG. 1A. The system includes laser source 2, storage medium 4, beam splitter 6, illumination optics 8, servo/data optics 10, detectors 12, and amplifier/decoder 14. In a reading process, a low-power laser beam from laser source 2 scans a data pattern on spinning storage medium 4, which may be a compact disk (CD). A reflected signal is directed with a beam splitter 6 to detectors 12. Detectors 12 produce a current signal, which is then decoded into user data. In a writing process, a higher power laser beam from source 2 writes a data pattern on spinning disk 4.
In the writing process, an input stream of digital information is converted with an encoder and a modulator (not shown) into an analog current drive signal for the laser. The ‘1’s in the drive signal switch the laser diode on and off alternatively. The intense light beam from the laser, when focused on the rotating scanning disc surface through the illumination optics, heats up the disc surface at the focused spot. The reflective property of the data layer at these regions is changed once the temperature goes beyond a threshold level. In this way, data are written on a spiral track around the center of the optical disc with alternating data marks and lands.
In the readout process, the laser is typically operated at a low, constant output power level that does not heat the medium, so that reflection is not affected by the laser beam. As the disc rotates, the reflected light is modulated upon reflection from the recorded data marks. The reflected light is then directed to detectors through servo/data optics and converted into an electrical detector current.
This detector current is a representation of the data pattern on the disc and is decoded into a digital signal. The detector current is a sinusoidal narrow-band radio frequency (RF) signal whose frequency for standard low speed (1×) CD readout varies from 196 kHz to 720 kHz. The detector current includes distinct frequencies corresponding to the nine possible lengths of data marks. This non-stationary signal exhibits a frequency content that varies randomly, depending on the occurrence of different runlengths. Further, the occurrence of defects on the surface of the disc cause sudden changes in the frequency content of the signal.
Data on a CD are written on a spiral track of alternating “data marks” and “lands.” The data marks and lands constitute an RLL data stream as shown in FIG. 1B with following constraints:
Minimum run-length constraint, d=2 (3T),
Maximum run-length constraint, k=10 (11T).
Thus, in this convention, there are nine run lengths for the data marks or lands (i.e., 3T, 4T, . . . 11T). Examples of basic units for the run lengths are shown in FIG. 1C.
Conventionally, a three-step modular approach in recovering data from damaged CD using microscope images has been used. In this approach, a readout signal is derived from the images. Then, data bytes are recovered from the signal. Finally, these bytes are arranged in a user-defined sequence. The conventional approach allows recovery from microscopic images and takes approximately 500 hours to recover data from a CD size area.
The following references whose contents in entirety are incorporated herein by reference represent background techniques and procedures used conventionally for data reading/writing/recovery:    Information technology—Data interchange on read-only 120 mm optical data discs (CD-ROM), ISO/EEC International Standard 10149, 2nd Ed.-1995.    S. Kasanavesi, T. D. Milster, D. Felix, T. Choi, “Data Recovery from a Compact Disc Fragment,” Proc. SPIE, 5777(1): pp. 116-127, September 2004.    T. D. Milster, “Optical Data Storage,” in The Optics Encyclopedia: Basic Foundations and Practical Applications, T. G. Brown, K. Creath, H. Kogelnik, M. A. Kriss, J. Shcmit, M. J. Weber, (eds.), Berlin:Wiley-VCH, 2004.    G. Strang, and T. Nguyen, Wavelets and Filter Banks, Wellesley-Cambridge Press, 1997    M. Vetterli, “Wavelets and Filter Banks: Theory and Design”, IEEE transactions on signal processing, Vol. 40, No. 9, September 1992.    M. Vetterli, “Filter Banks allowing Perfect Reconstruction”, Signal Processing 10 (1986), pp. 219-244.    A. F. Abdelnour, and I. W. Selesnick, “Nearly Symmetric Orthogonal Wavelet Bases”, Proc. IEEE Int. Conf. Acoust., Speech, Signal Processing (ICASSP), May 2001.    F. Li, “Data recovery from various damaged optical media”, Master's thesis, University of Arizona, Department of Electrical and Computer Engineering, 2005    A. Papoulis, and S. U. Pillai, Probability, random variables, and stochastic processes, 4th ed., New Delhi: Tata McGraw-Hill Publishing Company Limited, 2002, pp. 354-367.    D. L. Donoho, and I. M. Johnstone, “Adapting to Unknown Smoothness via Wavelet Shrinkage”, J. Am. Stat. Assoc., 90, 1200 (1995).